Accidents in the workplace cost many millions of dollars, hundreds of lives, and damage to the environment every year. The majority of these accidents are caused by human error. Human error has, of course, many causes, but it is most prevalent when an operator is impaired from lack of sleep, illness, or is under the influence of drugs or alcohol. In the U.S., 7.5 million workers in high-risk occupations are required to take random blood or urine tests to deter the use of drugs and alcohol on the job, a requirement which has helped reduce accidents. However, fatigue, illness and stress are more common causes of impairment than are the effects of drugs or alcohol. Accidents continue to occur in large part because workers are impaired by exhaustion, stress, side-effects from prescription medications or from a combination of these factors. There is therefore an urgent need for a way to screen workers for all impairment factors and causes before they begin work. Screening of this kind should be sensitive to impairment regardless of its cause, should be simple and quick, and should insure the maintenance of worker privacy.